Optical pyrometers are well known in the art having found extensive use in applications in severe environments or where temperature magnitudes prohibit the use of conventional contact pyrometric techniques. These devices calculate the temperature of a target from the radiant energy provided therefrom. An algorithm is used to determine the temperature of the surface by measuring total radiation in a given wavelength interval or by looking at the distribution of optical energy as a function of wavelength. The higher the temperature of the source, the greater the proportion of optical energy in the shorter wavelengths.
Optical pyrometers have been developed to measure the temperature of turbine blade surfaces even in an operating jet engine whose environment necessarily includes the combustion flame fireball. To accurately measure the turbine blade temperature, the optical pyrometer must be capable of correcting the measurements to eliminate the effect of the presence of reflected combustion flame radiation which is mixed in and obscures radiation from the turbine blade.
Dual spectral area optical pyrometers have been developed in order to differentiate between reflected and emitted radiation received from a target turbine blade and compensate for the error in the observed temperature that the reflected radiation introduces. In U.S. Pat. No 4,222,663, Gebhart et al. discloses dual band (two color) optical pyrometer which comprises two separate pyrometers. Each pyrometer sees a different but overlapping component of the total spectral range of the light or radiation from the turbine blade.
The pyrometers are sensitive at different wavelength bands and will be affected differently by the energy from the turbine blade surface. When the light (radiation) from the fireball is reflected off the blade, the pyrometer set to detect the shorter wavelength band is more responsive to the additional reflected energy, and its output signal increases in greater proportion than does that of the longer Wavelength pyrometer. Therefore, an increase or decrease in the amount of the reflected radiation or, at high reflection conditions, to the radiation temperature of the combustion flame will result in a proportionally higher or lower value of temperature indicated in the short wavelength pyrometer relative to the indicated temperature in the long Wavelength pyrometer.
For each pyrometer an algorithm calculates the temperature of the turbine blade from the light it receives. This process, which requires linearizing the relationship between the received power and temperature, is complex and degrades the temporal responsivity of the system. The linearized temperature signals indicate the equivalent blackbody temperature of the turbine blade. However, the reflected energy of the much hotter combustion flame will cause each of the two pyrometers to yield different temperature values, both higher than the true blade temperature. An additional temperature correction algorithm receives each channel temperature and determines the magnitude of the temperature error. The temperature correction signal is a function of the difference between the two pyrometer temperatures, which result from the spectral range of each pyrometer, the results from the spectral range of each pyrometer, the fireball equivalent blackbody temperature and the fraction of reflected radiation present in each pyrometer signal.
Reflection corrected radiosity optical pyrometers are disclosed in U.S. Pat. Nos. 4,708,474 and 4,222,663, each incorporated herein by reference, and generally include an optical guide for receiving from the target an optical beam that has a spectral width and has an emitted component from the target and a reflected component from a fireball that has an equivalent temperature. A detector module receives and divides the target optical beam into first and second optical beams and provides electrical signal equivalents thereof. The second optical beam is selected to have a spectral width that is a portion of the target beam spectral width. The pyrometer described in the '476 patent also includes a signal processor that provides for receiving the first and second signals as well as an energy ratio signal of the combustor fireball. The signal processor provides reflection corrected energy signals from a difference between the first signal and the product of the energy ratio signal and the second signal.
The dual spectral area pyrometers DSAP) found in the '663 discloses a reflection correction technique based on the principle that two blackbody calibrated pyrometers, each sensitive to different spectral wavelengths, will not respond equally when subjected to a radiative signal containing surface emitted and reflected radiation from a significantly higher temperature source. The pyrometer spectral ranges of operation are selected to provide sufficient sensitivity between the surface emitted radiation and the reflected component. The temperature difference between the two pyrometers is an indication of the magnitude of the reflected component.